The following meanings for the abbreviations used in this specification apply:    A/N, Ack/Nack Acknowledgement/Negative Acknowledgement    CA Carrier aggregation    CC Component carrier    CQI Channel quality indicator    CSI Channel state information    DL Downlink    DM Demodulation    eNB enhanced Node-B, LTE base station    FDD Frequency division duplexing    HARQ Hybrid automatic repeat request    LTE Long term evolution    PMI Precoder matrix indicator    PRACH Physical random access channel    PRB Physical resource block    PUCCH Physical uplink control channel    PUSCH Physical uplink shared channel    RF Radio frequency    RI Rank indicator    RS Reference signal    SC-FDMA Single carrier—frequency division multiple access    SR Scheduling request    SRS Sounding reference signal    TDM Time division multiplexing    UCI Uplink control information    UE User equipment    UL Uplink    X2 Standardized signalling interface between eNBs
Embodiments of the present invention relate to LTE-Advanced system which will most likely be part of LTE Rel. 11 or beyond. More specifically, we focus on uplink control signalling in the case of inter-site carrier aggregation having only single component carrier available in the UL direction. Carrier aggregation (CA) was introduced in Rel-10 of the E-UTRA specifications. By means of carrier aggregation (CA), two or more component carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz.
An exemplary deployment scenario is depicted in FIG. 1. FIG. 1 shows an example for inter site carrier aggregation, and shows in more detail a heterogeneous network (HetNet) scenario with two carriers. In particular, a macro eNB which serves a macro cell indicated by F1 (Frequency 1), and a pico eNB which serves a pico cell indicated by F2 (Frequency 2) are shown. A UE is connected to both eNBs. One of the carriers is allocated to macro layer and another for femto/pico layer, respectively. Another assumption is that there is a (logical) signalling entity (e.g. X2 interface) between macro and femto/pico, which can be used to convey control signalling between non-collocated cells. However, latency requirements for the control signalling are relatively relaxed.
Considering the Scenario in FIG. 1, as said, the two carriers (cells) operate independently to a large extent (scheduling etc). However, it would be beneficial to utilize some of the carrier aggregation functionality also in that case, to enable e.g. simultaneous DL data transmission on both carriers. This sets also some new requirements to the UL.
The problem is that Rel-10 carrier aggregation does not work in inter-site carrier aggregation scenario having independent L1/L2 schedulers operating on different sites. FIG. 2 below shows the PUCCH (Physical Uplink Control Channel) arrangement in the case of Rel-10 carrier aggregation. The basic principle is that the PUCCH is always located on only one UL CC (the primary CC) and all the UL control signalling (related to all DL cells) is carried over that particular UL CC (UL CC#2 in FIG. 2.). That is, in the example of FIG. 2, all the UL control signalling related to e.g. DL#2 and DL#3 are carried over a single PUCCH on UL#2.
However, in the scenario of interest this arrangement becomes infeasible, since it is clear that in this kind of scenario, the UL control signals (CQI/PMI/RI, HARQ ACK/NACK) corresponding to all aggregated DL component carriers need to be available at the same place (w/o delays). This is necessary to ensure that the two schedulers can operate fully independently.
From the signalling point of view the simplest way to cope with the issue is to have two UL in the UE operating somewhat autonomously, each of them transmitting the UL control signalling related to the corresponding DL CC. However, that solution would lead to rather complicated UE implementation, as it is far from trivial to include two full TX chains into the terminal with the capability to have them both transmitting simultaneously.
It was discussed already during Rel-10 standardization that it would be possible to define PUCCH/UCI functionality in a way that it consists of multiple symmetric DL/UL component carrier pairs (see e.g., R1-094642). This option is depicted in FIG. 3, which shows a potential UCI solution for intersite CA. In this solution UL/DL control signalling is made completely independent between among different component carriers.
The main problems of this approach are:                It does not support asymmetric carrier aggregation having only one component carrier available in UL side (this is the most important UE category with carrier aggregation)        UL coverage is an issue with this kind of arrangement. This is due to the fact that UL power control needs to work independently among two UL CCs. This results in a 3-dB coverage loss compared to Rel-10 intra-site CA+potentially further degradation due to power back-off required to fulfil necessary emission requirements.        